The invention relates to a method for determining, monitoring and updating correction data for correcting measured value distortions and for calibrating liquid-filled transmission systems.
Liquid-filled transmission systems may be used for invasive intra-arterial and intravenous pressure measurement in cardiology, intensive care medicine and anaesthesia. In these systems the pressure measurement takes place in the body of a patient and is transmitted via the liquid-filled transmission system formed as a catheter to a pressure transducer arranged outside the body of the patient. As a function of the length, cross-section, construction, the elastic material properties of the catheter and the composition of the liquid located in the catheter, various resonances, dampings and energy losses of the measured value of the pressure detected as a signal from the patient at the end of the catheter inside the body occur and lead to substantial distortions of this signal from the patient owing to the fluid-filled transmission system. These distortions do not allow any quantitative analysis of the signals from the patient and impair the qualitative interpretation of the diagnosis and monitoring.
To avoid distortions of the signals from the patient owing to the liquid-filled transmission system pressure measuring transducers known as tip pressure sensors were arranged at the tip of the catheter serving as the liquid-filled transmission system and the signal detected from the patient converted into an electrical signal and guided from the body of the patient via an electrical line. Tip pressure sensors of this type are very expensive, however, and only available to a limited extent with respect to their shape and size.
A method for computer correction of measured value distortions by the liquid-filled transmission system in the transmission of a signal from the patient measured inside the body of a patient is known from DE 1 982 208 844 A1. In this reference the electrical signal from the (distorted) signal from the patient emitted by the external pressure transducer is guided through an analogue/digital transducer and the digitalised signal emitted is analysed in a signal analysis and processing unit operating on the basis of a digital Fourier analysis and carrying out a beat by beat analysis of the digitalised signal. The analysed signal is then linked with empirically determined correction data called up from a correction data record matrix or as a correction data record vector and output as Fourier coefficients. The signal corrected by the signal analysis and processing unit is finally guided to an output and/or evaluation unit.
The correction data record required for this known method can be determined inter alia from a reference pressure measurement. In order to determine the transmission function, instead of the unknown measured signal, here an artificially generated known test pressure signal can be input as the reference signal, from the distortion of which the properties of the transmission system can then be concluded.
One possibility is to obtain a calibration data record from a calibrator arranged on the catheter tip, but this necessitates the maintenance of sterile conditions, causes awkward handling and means that changes in the transmission function have to be taken into account as, for example, the catheter guided to the pressure measuring site and calibrator is rinsed, medication is supplied via the catheter and so-called “microbubbles” change the transmission function. If such changes take place, recalibration is required which, however, is not easy to carry out in the case of a horizontally located catheter, in other words in a catheter located in the body of the patient.
A further possibility is to obtain a correction data record from the transmission function by means of an external calibration signal. For this purpose a calibration signal in the form of a jump signal, a pressure impact or a noise is transmitted at the side of the catheter remote from the measuring position, in other words outside the body of the patient, and the inherent oscillation produced in the process is used to calculate the correction data record. However, a plurality of problems occur in this type of determination of the transmission function by means of an external calibration signal. On the one hand, the calibration signal, i.e. the jump signal, the pressure impact or the noise have to be generated very precisely and in a reproducible manner and this necessitates measurements over a fairly long time period owing to the inconstant measuring behaviour of the transmission system and therefore cannot be implemented, for example in a hospital, owing to the high time and training input. On the other hand, use of an automatic mechanical device mounted to the pressure transducer, is very expensive and moreover, as a mechanical precision part, required special servicing for setting up and maintenance.
There is the additional problem of signal separation during calibration in a horizontal catheter, as otherwise the measured jump response to a jump signal by the actual signal from the patient, for example a blood pressure signal is too greatly distorted and therefore incorrect correction data records are determined. The resultant dependency of the instant of the resolution of the jump signal, impact on the system or noise substantially restricts the reproducibility and causes substantial effort in particular in the case of irregular signals from the patient.
A method for reducing the noise in an ECG signal is known from U.S. Pat. No. 5,827,195, in which a pulse sequence corresponding to the heartbeat is selected and is converted into a multi-dimensional display using a brief Fourier transformation for evaluating the time/frequency display. In addition, a multi-dimensional filter function is used on the multi-dimensional display of the pulse sequence to thus raise the signal-to-noise ratio of the pulse sequence. The statistical attempts used in the process to correct the ECG signal pulse sequence only serve to eliminate simple disturbances in the pulse sequence, however, which can be assumed to be normally distributed and of which the band width can easily be separated from the band width of the useful signal.